Typically, nipping elements are used to assist in the attachment of printing plates to the plate cylinders of a printing machine. In particular, nipping elements facilitate the attachment of a new printing plate to a cylinder by holding the new printing plate against the circumferential surface of the cylinder. Accordingly, nipping elements generally move between engaged and disengaged positions. In the engaged position the nipping element is urged against the circumferential surface of the cylinder thereby holding the new plate to the cylinder. In the disengaged position the nipping element is separated from the cylinder so as not to interfere with printing operations.
A nipping element of this type is known from DE 4,218,602 A1, in which the nipping element is designed as a roller that can be engaged against a cylinder in order both to facilitate insertion of a beveled end of the printing plate into a corresponding tension rail and to hold the printing plate to the circumferential surface of the cylinder during the attachment process. The nipping element is provided with a suspension system which moves the nipping roller between the engaged and disengaged positions. The suspension system includes a setting means and a spring which are arranged in series. The spring is provided in order to prevent injury to the operator of the printing machine in the event that his hand enters the gap between the disengaged nipping roller and the rotating cylinder. Thus, when an object, such as a hand, comes in contact with the disengaged nipping roller, the force generated by the contact causes the spring to compress thereby allowing the nipping roller to deflect away from the cylinder. The suspension system also includes a switching element which stops the printing machine when the disengaged nipping roller is deflected away the cylinder. However, the suspension system is not capable of holding the plate to the cylinder with a force large enough to ensure that the plate rests snugly on the surface of the cylinder because the setting element and the spring are arranged in series. Accordingly, in addition to allowing the disengaged nipping roller to deflect away from the cylinder, the compression of the spring helps the setting means generate the force used to hold the nipping roller against the cylinder when the nipping roller is in the engaged position. If a stiffer spring is provided in order to hold the plate more tightly against the cylinder it necessarily takes a greater force to deflect the disengaged nipping roller away from the cylinder. Therefore, a snug fit of the plates on the cylinder can be ensured only by compromising the safety of the operator.
DE 4,214,207 C1 discloses a nipping element comprising a nipping roller and a bent strip. The nipping roller and the bent strip are moved between the engaged and disengaged positions by a setting means. However, means are not provided to allow the disengaged nipping roller and bent strip to deflect away from the cylinder when they contact an object such as the hand of the operator. While the setting means are capable of holding the nipping roller and bent strip against the cylinder with a force sufficient to ensure a snug fit of the new plates on the cylinder, the design is disadvantageous because the problem of operator safety is not addressed.
DE 4,227,683 C2 discloses a nipping roller which is moved between the engaged and disengaged positions by a cylinder. Because the nipping roller and cylinder are not mounted to the side stand of the printing machine there is no apparent operator Safety problem that needs to be addressed.
JP 62-169646 A discloses a nipping element that includes a nipping roller which is moved between the engaged and disengaged positions by a pushing element. In addition, the nipping roller can move in the circumferential direction of the cylinder in order to facilitate the insertion of an end of the printing plate into a corresponding tension bar. However, the circumferential movement of the nipping roller introduces additional safety problems by increasing the number of places where the operator's hand may become caught.